1. Field of the Invention
The present invention relates to a diffusion sheet structure in the direct type backlight module with anti-UV rays function and its manufacturing method, which especially points at a diffusion sheet in the backlight module applied to TFT-LCD TV. This invention explores a diffusion sheet with a ultra-violet rays absorption layer avoid yellow and pyrolysis phenomenon and finally to improve product quality and materials lifespan.
2. Description of the Prior Art
Since TFT-LCD has been invented, all kinds of colour monitor have been widely applied in the daily life, such as notebook, PC monitor, colour mobile phone, PDA, and LCD TV etc., which have accordingly replaced conventional display such as CRT, TN and STN. An example of TFT-LCD TV, TFT-LCD TV being accordingly accepted by the consumers is because of liquid crystal panel producing technology improvement, such as short response time, high resolution, and higher yield of large panel. Besides, higher brightness of backlight module is also one of the main factors.
The larger size of TFT-LCD TV requires more, the higher brightness of one requires more. Normally the brightness of TFT-LCD TV requires 800˜1200 Cd/m2, moreover, the uniform backlight module technology through light guide panel by the traditional edge light source can not satisfy the requirements of TFT-LCD TV. Therefore, high brightness of direct type backlight source is occurred.
In order to enhance the brightness of TFT-LCD TV, it is necessary to increase the quantity of CCFL (Cold Cathode Fluorescent Lamp) and the edge light source is changed into direct type light source, shown as FIG. 1, which is a schematic diagram of the prior art of direct type backlight module. As FIG. 1 shown, said direct type backlight module 1′ comprises of a reflector 10′, plural light source 20′ and a diffusion sheet 30′. In order to avoid lamp mark, the light guide panel is required to be replaced by diffusion sheet to uniformly disperse the brightness of light source. The thickness of diffusion sheet is controlled in a range of 2 to 3 mm, the thickness of relative light guide panel is a range of 3 to 12 mm which having a large capability of reducing materials cost and to achieve the purpose of weight. The present diffusion sheet is mainly composed of PMMA (Polymethyl Methacrylate), MS (MMA/Styrene Copolymer), PC (Polycarbonate), and COC (Cycle Olefin Copolymer). and adding light diffusion particles in the transparent substrate to achieve uniform light.
The main strength of diffusion sheet in the composition of PMMA is well optical transmittance and yellow-resistance, however, the water absorption ratio of PMMA is in a range of 0.3 to 0.4% which resulted in a defect of warp and bad dimensional stability on the large TV. The light transmittance of MS and PC is small 2 to 3% than PMMA, but MS and PC has good performance on high temperature resistance and dimensional stability. The main defect is that MS and PC under irradiation of UV rays result in the problem of yellow. If the diffusion sheet in the composition of PC is long time irradiated under CCFL, then the problems of yellow will occur and result in colour temperature and chromatic aberration of panel. This problem is the main reason why diffusion sheet with PC materials cannot be extensively applied.
Therefore, providing a novel diffusion sheet structure in the direct type backlight module with anti-UV rays function and its manufacturing method for the aforesaid problems can not only improve the defects of yellow of the conventional diffusion sheet which resulted in deviation, but also can be simplified the process. Based on the inventor engaging on the research and develop and sales experiences for the related products for many years, the inventor finally proposes a method to improve the aforementioned problems for diffusion sheet structure in the direct type backlight module with anti-UV rays function and its manufacturing method, according to his professional background.